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465 Display Help Needed


Bill Wollam
 

Hello, I have been pulling problems out of a 465 that I recently
acquired. The last remaining problem is the inability to get a sharp
focus on the trace with any combination of intensity, focus or
astigmatism. I have checked everything in the CRT circuits with the
exception of the high voltage, crt bias and cathode bias, until I get
the use of an HV probe. Can anyone tell me if I am looking in the
right area or suggest any common failure modes? The low voltage
supplies all check good and noise free. Thanks for any help.


Kurt Graber <kurtg@...>
 

billw101@aol.com wrote:

Hello, I have been pulling problems out of a 465 that I recently
acquired. The last remaining problem is the inability to get a sharp
focus on the trace with any combination of intensity, focus or
astigmatism. I have checked everything in the CRT circuits with the
exception of the high voltage, crt bias and cathode bias, until I get
the use of an HV probe. Can anyone tell me if I am looking in the
right area or suggest any common failure modes? The low voltage
supplies all check good and noise free. Thanks for any help.

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A common problem or failure is the opening of the voltage divider circuit
that the focus and astig tap into.
I am unfamilier with your particular scope but the chances are good that one
of the resistors has opened up. I have seen many cases !!! you don't need a
HV probe , just take your meter and ohm them out. They will not appear burned
but they have a tendancy to open up due to the high voltage.......this will
cause an unsharp trace and many cases no trace at all (just a haze)...Good
Luck


dhuster@...
 

A few comments on what has been offered so far.

The characteristic fuzziness of a 465/475 and 7K scopes over their
earlier cousins (and which apparently is NOT your problem since you
describe major "focus" problems) is due to the inclusion of a scan
expansion mesh within the jug through which the electron beam has to
pass, causing a small amount of beam scatter. The mesh is there to
improve geometry.

If oscillations within the vertical system is the problem, then
inputting a sine wave will show a thicker trace at the peaks and a
thinner trace at the zero-crossing points. Also, you'll see the
signal from the vertical deflection plate pins on back to the source
of oscillation.

Open elements inside the jug can cause defocusing.

Don't forget that astigmatism and focus interact and one may be the
problem where the other is not. If you can "round up" an unfocused
stationary dot on the CRT with the astigmatism control, then focus is
your problem. If not, the astigmatism circuit may be at fault.

Make sure the LV power supplies are clean, correct and ripple-free.


Stan or Patricia Griffiths <w7ni@...>
 

CRT Mesh comment below:

dhuster@pb.k12.mo.us wrote:

A few comments on what has been offered so far.

The characteristic fuzziness of a 465/475 and 7K scopes over their
earlier cousins (and which apparently is NOT your problem since you
describe major "focus" problems) is due to the inclusion of a scan
expansion mesh within the jug through which the electron beam has to
pass, causing a small amount of beam scatter. The mesh is there to
improve geometry.
My understanding of the reason an expansion mesh was used in 465 and later
vintage tubes was really to improve the vertical and horizontal sensitivity
of the CRT such that expensive transistors would no longer be necessary in
the vertical and horizontal scope amplifiers. It was mostly a manufacturing
cost reduction measure with a spot size tradeoff. Not a good tradeoff, in my
opinion.

Compare the output stages of the 547, 453, and 454 scopes to what you see in
the 465 and 475 and you will see what I am talking about.

Stan
w7ni@easystreet.com